The Mesogranulation Convective Power Spectrum

Authors: Laurence J. November

Some authors have pointed out that the observed spectrum of long-lived solar horizontal velocity shows only a single peak at wavelength ≃35 Mm, ‘supergranulation’. However the corresponding verticalvelocity spectrum looks very different with power shifted to higher wavenumber in a broad divided peak representing a range of plume sizes or half wavelengths from 4–12 Mm, i.e. ‘mesogranulation’. Vertical-velocity spectra derived from the Hathaway et al. (2000) SOHO-MDI 62 day full-disk Dopplervelocity spectrum and based upon the Koutchmy (1994) granulation intensity spectrum show expected Kolmogorov-inertial and eddy-noise power-law wavenumber subranges, giving evidence for energy injection into the vertical flow around three coherent length scales &1.01, 5.4, and 10.5 Mm, i.e. granulation, and ‘mesogranular’ and ‘supergranular’ subsurface counterparts. The three energy-injection scales correspond reasonably to interior-model convective mixing lengths for H I, He I, and He II 50% ionization depths, respectively. The two larger scales are nearly one-to-two also suggestive of a possible resonant overtone structure. The horizontal supergranulation flow seems evident as a distinct scale of eddy noise without energy injection around wavelength 45.3 Mm, consistent with the first subharmonic of the supergranular subsurface counterpart.

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[v1] 2018-07-03 16:00:14

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